It is well known to package explosive compositions in purpose designed packages to assist in transportation and handling of explosives compositions including dry powder or granulated compounds, emulsions, slurries, water gels and the like.
These packages may be adapted to suit charge mass, borehole diameter, borehole length and the like.
Australian Patent Application AU-A-81802/87 describes the packaging, in non-porous plastic cartridges of from 25 mm to 75 mm in diameter and 1000 mm length, an explosive composition comprising ammonium nitrate, paraffinic oil and foamed or expanded polystyrene beads.
British Patent 882665 describes the packaging of ammonium nitrate compositions in paper wrapped cartridges.
British Patent 1281421 also describes packaging of an ammonium nitrate explosive in a thin flexible plastic sheath such as polyethylene. This product has detonating cord extending throughout an elongate explosive filled sheath in a unitary charge of up to 20 m long. The specification discloses divisible charges in rolls of from 50 m to 100 m in length whereby charges of selected shorter length can be formed by clamping the sheath at closely spaced intervals and severing the charge therebetween.
The patent specification states that explosive charges according to the invention can be inserted into boreholes at up to 60° from vertical and moreover that with the aid of a bracket shaped slide, these packages can be inserted into horizontal boreholes.
Experience has shown however that thin flexible sheaths of say 0.2 mm in wall thickness are prone to perforation by puncture on rough borehole surfaces or by abrasion even on smooth borehole surfaces. If such perforations occur near the lower end of the charge being inserted, there is a risk that a substantial part of the freely flowing particulate explosive composition would fall to the bottom of the borehole beyond the end of the detonating cord without the knowledge of the personnel concerned. Only when the connected charges in adjacent boreholes were detonated would it be discovered that the mechanical and safety aspects of the integrated breaking pattern were compromised by the detonation failures in one or more boreholes.
These soft thin walled explosive packages were found to be difficult and time consuming to insert in to horizontal boreholes and even more prone to puncture and tearing when pushed into the horizontal cavity.
A particular difficulty with such flexible, thin walled elongate explosive charges is that when rolled into conveniently sized coils for packaging, transportation and storage, the tubular sheath is prone to kinking or flattening in parts.
At the position of the kink or flattened region, the cross sectional area of the explosive charge is substantially diminished and with severe kinking, a discontinuity may be formed in the mass of explosive material. This kinking or cross sectional area reduction can also occur when trying to insert elongate thin walled flexible tubular explosive charges into inclined or horizontal boreholes.
It is believed that severe reductions in cross sectional area or discontinuity in the mass of explosive composition may give rise to a condition known as “gapping” where propagation of explosive energy along the charge column is interrupted.
In order to overcome such problems with prior art packaged explosives, particularly for use in perimeter hole blasting for tunnel shaping where the boreholes are horizontally drilled, it has been proposed to use short, ridged tubular packages adapted for end to end coupling to form a long unitary charge.
Originally such explosive packages comprised a stiff paper or cardboard cylinder about 900 mm long and about 20 mm diameter filled with highly viscous, non flowable cap sensitive emulsion explosive. The paper or cardboard cylinder was open at opposite ends which were shaped to form a socket and spigot engagement between adjacent packages.
Of more recent times these paper or cardboard cylinders have been replaced with a semi rigid plastic tube such as polyethylene with wall thickness of about 1 mm.
A difficulty encountered with these prior art modular explosives tubes is that a close face to face abutment of the explosive composition in adjacent tubes is required to ensure propagation of the explosive force throughout the entire assembly which may for example comprise six tubes frictionally engaged by the socket and spigot fittings.
As typically these tubes are initially filled with a heated mix to reduce viscosity of the normal non-flowable viscous emulsion, the tube contents undergo shrinkage on cooling and can create a gap of 2–3 mm between the faces of explosive material when the tubes are joined.
Where “gapping” occurs between adjacent explosive masses, whether due to shrinkage in the tube or human error in assembly, an incomplete explosion can leave one or more portions of the explosive charge in the ground. This is an extremely hazardous situation with cap sensitive explosives in particular, which may be accidentally detonated by impact from an excavator or subsequently in a crushing mill.
Another problem associated with prior art blasting techniques is that where it is required to increase the powder factor or energy factor in the “butt” or base of a borehole, it is common practice to first push a separate primer charge to the butt of the borehole.
Thereafter, a water gel or emulsion explosive is loaded into the borehole typically in a form of a spigot and socket joined tube described above.
Again this procedure is prone to unreliability due gapping between the primer and the end of the explosive charge and/or between adjacent tubes which can lead to ineffectual blasting, rifling of the borehole and unexploded explosives left in the borehole.
Yet another disadvantage associated with abovementioned prior art explosive charge is the need to insert a stem plug to close off the borehole after the charge is loaded. This a costly and time consuming process.
Australian Patent 742653 to the same inventor describes an elongate coilable blasting cartridge having a detonating cord extending longitudinally within the tubular casing and terminating in a distal end of enlarged diameter compared with the main part of the tubular casing. This product is available in 4–5 meter lengths and addresses many of the prior art problems referred to above.
Although the coilable elongate blasting cartridge described in Australian Patent No 742653 is not prone to gapping, there exists a need for a modular blasting cartridge construction capable of forming variable cartridge lengths as required but which still has the reliability of the product of Australian Patent No 742653.